In a liquid crystal display device, the liquid crystal is a substance that does not itself radiate light. Instead, the liquid crystal relies on receiving light from a light source in order to display images and data. In a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light.
Referring to FIG. 18, a typical backlight module includes a light source 10, a light guide device 12, and a plurality of optical correcting elements. The light guide device 12 includes an incident surface 122 adjacent to the light source 10, an emitting surface 124 located at the top surface of the light guide device 12 and adjacent to the incident surface 122, and a reflecting surface 126 opposite to the emitting surface 124. The optical correcting elements include a reflective sheet 11 positioned under the reflecting surface 126 for reflecting light back into the light guide device 12 again, a diffusion sheet 13 positioned above the emitting surface 124 for diffusing emitted light and thereby avoiding a plurality of bright sections in the light guide device 12, and a brightness enhancement sheet 14 positioned above the diffusion sheet 13 for collimating the emitted light beams uniformly to improve the brightness. However, the optical correcting elements make the backlight module quite complicated and costly to manufacture.
The light guide device 12 converts the light source 10 into a surface light source, and is one of the key components of the backlight module. Generally, the light guide device 12 does not have a function of controlling the direction of light emitted therefrom. When the light source 10 emits a light 101, the light guide device 12 receives the light 101 via the incident surface 122, reflects the light 101 at the reflecting surface 126, and emits the light 101 from the emitting surface 124 in an oblique direction away the light source 10. The angle of emission is not in a direction perpendicular to the emitting surface 124. Therefore a plurality of optical correcting elements needs to be added to and matched with the light guide device 12, for controlling the emitting light beams to emit perpendicularly toward a liquid crystal plate (not shown).
Uniformly emitting light and uniform light emitting angles are considered to be the important design considerations of a light guide device. If a light guide device can control a plurality of the emitting light beams to emit uniformly and perpendicular to the emitting surface thereof, or to emit within an appropriate light emitting angle range, the light guide device can realize the function of the conventional backlight module without the need for adding a plurality of optical correcting elements to the backlight module.
A conventional backlight module which can control the light emitting angle is shown as FIG. 19. The backlight module includes a light source 40, a light guide device 47 having a incident surface 49, and a transparent reflecting means 48 in optical contact with the light guide device 47. The reflecting means 48 includes an optional adhesion promoting layer 46, and an array of microprisms 45 formed on the layer 46. Light reflects through the light guide device 47 via total internal reflection, enters the microprisms 45 by way of light input surfaces 41 thereof, reflects off sidewalls 42 of the microprisms 45, and exits the microprisms 45 through emitting surfaces 43 thereof as a spatially directed light source. However, the reflecting means 48 make the backlight module high rather complicated in structure and costly to manufacture. In particular, the light guide device 47 is difficult to mass produce by way of mold injection technology.
A conventional light guide device is shown in FIG. 20. The light guide device 50 includes a body for guiding light transmitted from a light source 56. The body has a plurality of recesses 54 formed on a bottom surface thereof, and a plurality of light collecting grooves 52 formed in a direction parallel to the light source 56 on a top surface thereof. Each of the recesses 54 defines a polyhedral horn shape. Each of the grooves 52 has a V-shape in a sectional view that is taken along a direction parallel to the light source 56. Respective sizes of the recesses 54 increase with increasing distance from the light source 56, and respective distances between adjacent recesses 54 decrease with increasing distance from the light source 56. The plurality of recesses 54 on the bottom surface of the light guide device 50 and the plurality of V-shaped grooves 52 on the top surface of the light guide 50 can improve a brightness of the light and reduce a range of emitting angles of the light. However, the light guide device 50 cannot control a plurality of the emitting light beams to emit uniformly and perpendicular to the top surface of the light guide device 50. Therefore, the light guide device 50 generally still needs to have a plurality of optical correcting elements to be added in order to realize the desired function of a conventional backlight module.
What is needed, therefore, is a light guide device which can control a plurality of emitting light beams to emit uniformly and perpendicular to an emitting surface thereof, and which can realize the function of a conventional backlight module without having to add a plurality of optical correcting elements thereto.